During the past year, the Laboratory of Immunology has evaluated the biochemical and immunological properties of three M. intracellulare antigens and has completed the nucleotide sequences of the genes encoding these antigens. MI22- Nucleotide sequence analysis demonstrated that MI22 is a homolog of the serologically active 19 kDa M. tuberculosis protein. Purified MI22 is recognized by 70% of sera from tuberculosis patients and 45% of sera from AIDS patients with MAC disease. Genetic and biochemical characterization suggest that MI22 is a lipoprotein. MI43- Nucleotide sequence analysis indicates that the MI43 gene encodes a 27 kDa protein. Serological assays have demonstrated that 90% of sera from tuberculosis patients and 50% of AIDS patient sera react with this protein. Genetic and biochemical analyses suggest that MI43 is a lipoprotein. MI85- Computer-based comparisons of the MI85 gene sequence to known gene sequences demonstrated that MI85 has 70% protein sequence identity to the an Escherichia coli peroxidase-catalase and 60% protein sequence identity to a Salmonella catalase. In addition, E. coli lysates containing overexpressed MI85 protein exhibited significant peroxidase and catalase enzymatic activity. These genetic and biochemical results indicate that the MI85 encodes a peroxidase-catalase, an enzyme which is critical for the intraphagosomal survival of mycobacteria.